Dyed and vacuum-coated plastic films are applied to windows either to reduce glare or to reduce heat load due to sunlight. To reduce glare, the transmission of visible light (T.sub.VIS) at wavelengths between 400 nm and 700 nm must be controlled. To reduce heat load, solar transmission (T.sub.SOL) is blocked in either the visible or the near-infrared (NIR) portions of the solar spectrum (i.e., at wavelengths ranging from 400 nm to 2100 nm).
Primarily through absorption, dyed films can control the transmission of visible light, T.sub.VIS, to any level desired and consequently afford excellent glare reduction. However, dyed films generally do not block near-infrared solar energy and consequently are not completely effective as solar control films. Another shortcoming of dyed films is that they often fade with solar exposure. In addition, when films are colored with multiple dyes, the dyes often fade at different rates, causing unwanted color changes over the life of the film.
Other known window films are fabricated using vacuum-deposited grey metals, such as stainless steel, inconel, monel, chrome, or nichrome alloys. The deposited grey metal films offer about the same degrees of transmission in the visible and near-infrared portions of the solar spectrum. As a result, the grey metal films are an improvement over dyed films with regard to solar control. The grey metal films are relatively stable when exposed to light, oxygen, and/or moisture, and in those cases in which the transmission of the coatings increases due to oxidation, color changes are generally not detectable. After application to clear float glass, grey metals block light transmission by approximately equal amounts of solar reflection and absorption.
Vacuum-deposited layers such as silver, aluminum, and copper control solar radiation primarily by reflection and are useful only in a limited number of applications due to the high level of visible reflectance (R.sub.VIS). A modest degree of selectivity (i.e., higher visible transmission, T.sub.VIS, than near-infrared transmission) is afforded by certain reflective materials, such as copper and silver.
Traditionally, the best glare reducing coatings have been sputtered grey metals such as stainless steel, chrome, or nickel. The graph of FIG. 1 is a transmission spectra 10 for a sputtered nichrome coating that is designed to transmit approximately 50% of the light at the center of the visible light spectrum. The nichrome film is affixed to a 3.2 mm thick plate of float glass. As can be seen, the transmission of energy is controlled in both the visible (400 nm-700 nm) and near-infrared (700 nm-2100 nm) portions of the solar spectra. A slight degree of wavelength selectivity is observed due to the iron oxide in the glass.
In the graph of FIG. 2, the visible reflectivities R.sub.VIS of single and double layer nichrome films of various thicknesses are shown as a function of the corresponding visible light transmission T.sub.VIS. The data for the graph of FIG. 2 is found in Table 1. Here, the double nichrome films refer to a construct in which two optically isolated sputtered coatings are employed. The two coatings are separated from each other by a relatively thick (i.e., greater than 2 micrometers) layer such as a laminating adhesive. Referring to FIG. 2, the nichrome layer thicknesses decrease from left to right, and as can be seen, as the nichrome layers get thinner the reflection of visible light, R.sub.VIS, decreases and the transmission of visible light, T.sub.VIS, increases. The comparison between the single and double layer nichrome films shows that the double layer of nichrome has a substantially reduced R.sub.VIS for the same T.sub.VIS. For example, at a T.sub.VIS of 20% the single nichrome coating has an R.sub.VIS of 24%, while the double nichrome coating has an R.sub.VIS of 13%. As the nichrome layers get thinner, the R.sub.VIS of the two films converge.
The degrees of solar rejection achieved by films with single and double layers of nichrome are compared in the graph of FIG. 3. Solar rejection is defined as: EQU % solar rejection=solar reflection+(0.73.times.solar absorption).
Within the art, solar rejection is often calculated according to the ASTM E424B method. The slightly better solar rejection noted for the low transmission single nichrome coatings relative to the twin nichrome equivalents is due to solar reflection differences.
A low visible light transmission, T.sub.VIS, and low visible light reflection, R.sub.VIS, film utilizing double layers of nichrome is disclosed in U.S. Pat. No. 5,513,040, entitled "Optical device having low visual light transmission and low visible light reflection", issued to Yang. Yang discloses a solar control film having two or more transparent substrates, each bearing a thin, transparent, discontinuous, incoherent film of metal, having low R.sub.VIS and a degree of visible light blocking capacity. The substrates are arranged and laminated into a composite such that the visible light blocking capacities of the metal films are effectively combined to provide a composite having low visible light transmittance, T.sub.VIS Referring to FIG. 4, Yang specifically describes a film that is affixed to a glass window 20 comprising, in order from top to bottom, a pressure-sensitive adhesive layer 22, a polyethylene terephthalate layer 24, a nichrome layer 26, an adhesive layer 28, a nichrome layer 30, a polyethylene terephthalate layer 32, and a hardcoat layer 34.
A known film used to provide solar control is the silver-based Fabry-Perot interference filter (Fabry-Perot filter). Fabry-Perot filters provide good solar control because the filters have a high degree of wavelength selectivity. For example, using a Fabry-Perot filter, visible light can be transmitted at about 70% while near-infrared solar radiation is transmitted at less than 10%. Coatings utilizing Fabry-Perot filters are aesthetically acceptable in that the visible reflection R.sub.VIS of laminated glass structures containing such films can be very low, generally near 10%. An example of the use of a Fabry-Perot interference filter for solar control is disclosed in U.S. Pat. No. 5,111,329, entitled "Solar Load Reduction Panel with Controllable Light Transparency", issued to Gajewski et al.
In applications in which solar control is desired with a minimal effect on the visible optical properties of the window, Fabry-Perot filters are desirable. However, in applications in which glare control is desired as well as solar control, the high visible transmission of Fabry-Perot filters makes them unacceptable. As a result, what is needed is an improved solar control film that has low visible light transmittance and low visible light reflectance.